(1) Field of the Invention
The invention relates to antennas and is directed more particularly to a design for an open Yaggi antenna array.
(2) Description of the Prior Art
Most prior art Yaggi antennas consist of a driven element and two or more non-driven elements. The driven element is often a half-wave dipole. It is arranged in front of and parallel to a non-driven element that serves as a reflector. The driven element is also arranged behind and parallel to an array of one or more other parasitic elements that serve as directors. The reflector reflects radiation from the dipole back toward the dipole. The directors narrow the dipole radiation along the director side of the dipole. Both the driven and non-driven elements are all parallel on an axis along the same spatial plane.
The resultant radiation pattern of the Yaggi antenna as described above is a relatively narrow unidirectional beam along the direction of the director elements away from the dipole. The narrow beam effect produced by the reflector and directors occurs over approximately a 15% bandwidth about the half wavelength frequency of the dipole.
There are certain problems with the Yaggi antenna as described above. In particular, the reflector and directors have various undesirable effects on the original impedance of the dipole. The reflector and directors cause a “shunting effect” on the dipole, resulting in reduced antenna impedance in the region where the antenna operates, (at or near 0.5 wavelengths resonance). In addition, the reflector and directors also cause a decrease in the impedance bandwidth of the antenna. Since the directors are parasitic elements, they introduce undesirable resonance/anti-resonance loops in the original impedance of the dipole. What is needed, therefore, is a Yaggi antenna array design that avoids the shunting effect caused by the reflector element and parasitic director elements on the driven element.